I have a 1983 Westy with 2012 GoWesty replacement springs (normal lift).
Almost from the time of installation I noticed the car wasn't sitting
level, presumably because of the imbalance caused by the westy interior and
camping equipment.

The sag hasn't changed much in 6 years. Here's what it looks like. The
measurements are from the lip of the wheel arches to the center of the hubs:

Driver front: 16.5 inches
Passenger front: 17
Driver rear: 16
Passenger rear 16.75

I realize that you can't shim these springs like they were mechanical cam
lifters, but that they are means of racking the car around the center of
gravity as a fulcrum, and in the front the suspension geometry has some
mechanical gain to it affected by the height changes in the rear. So, I'm
not going to try to learn all that for what I hope is a one-time fix.

My question is a) what are the best options for shimming and b) where to
get the shims?

Thanks in advance to those who know.

Jim